Final answer:
The competitive exclusion principle states that different species cannot coexist if they are competing for the same resources. This principle works because natural selection favors traits that reduce competition. When a keystone species is removed from a community, it can have significant effects on the other species and the overall ecosystem.
Step-by-step explanation:
The competitive exclusion principle states that two species cannot occupy the same ecological niche in a habitat. This means that different species cannot coexist in a community if they are competing for all the same resources. This principle works because competition wastes energy, so natural selection favors traits that reduce competition. If one species cannot evolve to reduce competition, then the more efficient species will drive the other species to extinction. For example, when two protozoan species, Paramecium aurelia and Paramecium caudatum, are placed together in the same habitat, P. aurelia outcompetes P. caudatum for food, leading to the latter's extinction.
In a community, a keystone species is one that has a disproportionately large impact on the ecosystem compared to its abundance. When a keystone species is removed from a community, it can have profound effects on the other species and the overall ecosystem. For example, jaguars are a keystone species in the Amazon rainforest. Despite being less abundant than many other species, they play a crucial role in regulating the populations of their prey, such as deer and peccaries. If jaguars were to be removed, the prey populations could increase unchecked, leading to overgrazing and the depletion of resources for other species. This could have cascading effects on the entire ecosystem, affecting the abundance and distribution of other species and altering important ecological processes.